psad (Intrusion Detection and Log Analysis with iptables)

psad is a collection of three lightweight system daemons (two main
daemons and one helper daemon) that run on Linux machines and analyze
iptables log messages to detect port scans and other suspicious traffic.
A typical deployment is to run psad on the iptables firewall where it has
the fastest access to log data.

You can set email for alerts by setting ALERTSEMAIL:

ALERTSEMAIL=alerts@example.com ./psad.SlackBuild

You need at least these rules:

iptables -A INPUT -j LOG
iptables -A FORWARD -j LOG

but more usefull will be something like this:

iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -j LOG
iptables -A INPUT -j DROP

please see documentation for more information.